(1) Field of the Invention
This invention relates relates to a method for constructing a hologram which can be used, for example, in a hologram scanner applied, for example, to a point-of-sale (POS) system.
(2) Description of the Prior Art
POS systems have come into increasing use in supermarkets, department stores, and other commercial sales outlets in recent years. Use of a POS system enables computer monitoring of a large quantity and variety of goods and, accordingly, automatic calculation and classification of the sales and supply of those goods. In POS systems, information concerning each of the goods is marked directly on the goods in advance by attaching a bar-code label. The bar-codes can be read and detected by the aid of holograms.
A hologram is constructed by optical interference of two coherent light beams. More specifically, a laser source provides a laser beam. The laser beam is separated into two beams, for example, by means of a beam splitter or a half mirror. The first laser beam is, for example, a plane wave (so-called reference wave). The second beam is, for example, a spherical wave (so-called objective wave). These two construction laser beams impinge on a photosensitive layer coated on a transparent glass plate (base plate) at different incident angles. As a result, interference fringes are produced in the photosensitive layer. The interference fringes form so-called diffraction gratings.
In reconstructing a hologram as constructed above, for example, for the purpose of scanning, or copying it, one usually uses reconstruction laser beams having the same incident angle as that of one of the afore-mentioned construction laser beams. Use of identical laser beams results in maximum diffraction efficiency of the beams since the incident angles of the reconstruction beams are thus always Bragg angles with respect to the inclined angles of interference fringe planes of the hologram at any point corresponding to the fringe planes. Use of identical reconstruction beams, however, also results in the problem of large aberration of the wavefront obtained by the reconstruction of the hologram.
To decrease aberration, the assignee of this application previously proposed in U.S. Pat. No. 4,235,504 to reconstruct a hologram by using reconstruction beams having wavefronts different from those of the construction beams. However, the use of such reconstruction beams different from the construction beams resulted in a decreased diffraction efficiency. That is, it was not possible to decrease the aberration and maintain a high diffraction efficiency at the same time.